1. Field of the Invention
The present invention relates to metal halide lamps and lighting methods for the same.
2. Related Art
A metal halide lamp includes an arc tube formed from a discharge vessel having a discharge space therein and constituted from a main tube and two thin tubes that extend one from each end of the main tube, and a pair of electrode inductors that are sealed one within each thin tube so that respective ends of the inductors are opposed in the discharge space. A light-emitting material, a buffer gas, and a starting rare gas are enclosed within the arc tube. The light-emitting material is formed from halides such as dysprosium iodide (DyI3), thulium iodide (TmI3), holmium iodide (HoI3), thallium iodide (TlI) and the like, while the buffer gas is formed from mercury and the starting rare gas is formed from argon and the like.
The discharge vessel is made from a translucent ceramic material, since this increases the heat resistance of the arc tube above that of conventional arc tubes made from quartz glass, and also because of the favorable lamp properties that are obtained such as high lamp efficiency, high color rendering, and long life.
In recent years, investigations into the operation under dimming conditions of metal halide lamps using a ceramic discharge vessel have been conducted with a view to reducing energy consumption. However, when a metal halide lamp with a ceramic discharge vessel is operated at low lamp power under dimming conditions, the lamp properties deteriorate markedly in comparison to operation at high lamp power, making the lamp impractical. Note that in the present invention, a lamp is defined as being impractical if either the color temperature differential or the Duv (deviation from blackbody locus×1000) differential under dimming conditions at minimum and maximum lamp power is ≧750 K and ≧7, respectively.
That is, the concentration of light-emitting material, buffer gas, argon and the like enclosed in the arc tube is designed for dimmed lighting at high lamp power. Thus, the vapor pressures of the halides (TlI, DyI3, TmI3, HoI3) constituting the light-emitting material when the lamp is operated at high lamp power are well balanced, allowing an ideal emission spectrum to be obtained. However, when the lamp is operated at low lamp power, there is only a slight reduction in the vapor pressure of TlI in contrast to the marked drop in the vapor pressures of the rare-earth metal iodides (DyI3, TmI3, HoI3). Consequently, the strong emission spectrum obtained for the Tl emission causes a change in color temperature, while the weaker emission spectrums obtained for the rare earth metals cause a deterioration in lamp efficiency.
In view of this, a metal halide lamp has been proposed that equalizes the drop in vapor pressure of the halides during low lamp power operation by replacing TlI (exhibiting only a slight drop in vapor pressure under low lamp power conditions) with MgI2 (magnesium iodide), which has almost the same vapor pressure change as rare-earth metal halides such as DyI3, TmI3, HoI3. This enables excellent color rendering to be obtained even when the lamp is operated at low lamp power (see, for example, Japanese Published Patent Application No. 2002-42728).
However, with metal halide lamps having MgI2 enclosed therein, lamp life is shortened due to the high reactivity of the MgI2 with the translucent ceramic material constituting the discharge vessel, making these lamps not really practical for dimmed lighting.